With economic development, terrain in rugged natural environments that have hitherto not been developed is being mined for natural resources such as coal. Therefore, the track environment of a freight railway for transport of resources has become significantly harsher, and thus there is demand of the rail for wear resistance, and toughness in cold regions, and the like at least as high as currently available. From this background, there is demand for the development of a rail having wear resistance and high toughness at least as high as the high-strength rail that is currently used.
In order to improve the wear resistance of rail steel, rails as described below were developed. The main characteristics of such rails are that in order to enhance wear resistance, the carbon content in steel was increased, the volume ratio of a cementite phase in pearlite lamellae was increased, and moreover hardness was controlled (for example, refer to Patent Documents 1 and 2).
In the technique disclosed in Patent Document 1, using hypereutectoid steel (with higher than 0.85% to 1.20% of C), the volume ratio of cementite in the lamellae in a pearlite structure is increased, thereby providing a rail having excellent wear resistance.
In addition, in the technique disclosed in Patent Document 2, using hypereutectoid steel (with higher than 0.85% to 1.20% of C), the volume ratio of cementite in the lamellae in a pearlite structure is increased, and simultaneously, hardness is controlled, thereby providing a rail having excellent wear resistance.
In the techniques disclosed in Patent Documents 1 and 2, the volume ratio of the cementite phase in the pearlite structure is increased by increasing the carbon content in steel, and thus an increase in wear resistance to a certain level is achieved. However, in such cases, the toughness of the pearlite structure itself is significantly degraded, and thus there is a problem in that rail breakage is likely to occur.
From this background, it was desired to provide a steel rail having excellent wear resistance and toughness obtained by enhancing the wear resistance of a pearlite structure and simultaneously enhancing toughness.
In general, in order to increase the toughness of pearlite steel, it is said that refinement (increasing the fineness) of a pearlite structure, specifically, refinement of the grains of an austenite structure before pearlite transformation or refinement of a pearlite block size is effective. In order to achieve the fine-grained austenite structure, a reduction in rolling temperature and an increase in rolling reduction during hot rolling, and moreover, heat treatment by low-temperature reheating after rail rolling, are performed. In addition, in order to achieve the fine pearlite structure, acceleration of pearlite transformation from the inside of austenite grains using transformation nuclei, or the like is performed.
However, in the manufacture of rails, from the viewpoint of ensuring formability during hot rolling, there are limitations on the reduction in rolling temperature and the increase in rolling reduction, and thus sufficiently refinement of the austenite grains is difficult to achieve. In addition, regarding the pearlite transformation from the inside of the austenite grains using the transformation nuclei, there are problems in that controlling the amount of transformation nuclei is difficult, the pearlite transformation from the inside of the grains is not stabilized, and the like, preventing a sufficiently fine pearlite structure from being achieved.
From these problems, in order to fundamentally improve the toughness of a rail having a pearlite structure, a method of performing low-temperature reheating after rail rolling, and thereafter causing pearlite transformation through accelerated cooling, thereby refinement of the pearlite structure has been used. However, in recent years, there has been a progressive increase in the carbon content in rails in order to improve wear resistance. In this case, there is a problem in that coarse carbides remain dissolved in austenite grains during the low-temperature reheating heat treatment, and thus the ductility or toughness of the pearlite structure is degraded after the accelerated cooling. In addition, since the reheating is performed, there are economic problems such as high manufacturing cost and low productivity.
Here, there is demand for the development of a method of manufacturing a high-carbon steel rail by ensuring formability during hot rolling and refinement of a pearlite structure after the hot rolling. In order to solve the problems, methods of manufacturing a high-carbon steel rail as described below have been developed. The main characteristics of such rails are that in order to increase the fineness of a pearlite structure, a property of austenite grains of high-carbon steel being more likely to recrystallize at a relatively low temperature and at a small rolling reduction amount is used. Accordingly, well-ordered fine grains are obtained by continuous rolling with a small rolling reduction, thereby enhancing the ductility or toughness of pearlite steel (for example, refer to Patent Documents 3, 4, and 5).
In the technique disclosed in Patent Document 3, in finish rolling of a steel rail having high-carbon steel, three or more continuous passes of hot rolling are performed between predetermined interval time of rolling passes, thereby providing a high-ductility and high-toughness rail.
In addition, in the technique disclosed in Patent Document 4, in finish rolling of a steel rail having high-carbon steel, two or more continuous passes of rolling are performed between predetermined interval time of hot rolling passes, and moreover, after performing continuous rolling, accelerated cooling is performed after the hot rolling, thereby providing a high-wear-resistance and high-toughness rail.
Moreover, in the technique disclosed in Patent Document 5, in finish rolling of a steel rail having high-carbon steel, cooling is performed between hot rolling passes, and after performing continuous rolling, accelerated cooling is performed after the hot rolling, thereby providing a high-wear-resistance and high-toughness rail.
In the techniques disclosed in Patent Documents 3 to 5, by the temperature during continuous hot rolling, and a combination of the number of rolling passes and time between passes, refinement of the austenite structure to a certain level is achieved, and thus a slight increase in toughness is acknowledged. However, the effect is not acknowledged regarding fractures that occur from inclusions existing in steel as origins or fractures that occur from a pearlite structure as an origin other than from inclusions as origins, and toughness is not fundamentally enhanced.